1. Field of the Invention
The present invention relates to a metal-organic framework composite including a host metal-organic framework and nano metal-organic frameworks embedded in the host metal-organic framework, a method for producing the metal-organic framework composite, and a gas storage using the metal-organic framework composite. More particularly, the present invention relates to the use of a host metal-organic framework and nano metal-organic frameworks including different metals and organic ligands, the production of a metal-organic framework composite in which the nano metal-organic frameworks are embedded in the host metal-organic framework, and the application of the metal-organic framework composite to a gas storage.
2. Description of the Related Art
Metal-organic frameworks were first reported by Omar M. Yaghi, a professor of the University of California, Berkeley (USA) (Non-patent Document 1). Metal-organic frameworks are synthesized from metal precursors and organic ligands as linkers in a particular solvent by a hydrothermal method and have a three-dimensional porous structure in which the metal blocks and the organic ligands are repeatedly arranged. Metal-organic frameworks have a very large specific surface area due to the presence of micropores or mesopores. Metal-organic frameworks have received attention as materials for gas storage disc to their large specific surface area. Another advantage of metal-organic frameworks is that metal precursors can be very freely combined with organic ligands. Due to these advantages, thousands of metal-organic frameworks have been registered in the database so far.
Metal-organic frameworks were synthesized using a single metal and a single organic ligand in the early days (Patent Document 1). Since then, methods have been reported for the synthesis of various organic ligands to adjust the pore size of metal-organic frameworks. There have also been a number of reports on methods for the synthesis of metal-organic frameworks suitable for the manufacture of more effective gas storages by mixing many metals with different organic ligands.
Furthermore, a metal-organic framework having a functionalized organic ligand capable of promoting a catalytic reaction was synthesized (Non-patent Document 2). The functionalization of the organic ligand can maximize the catalytic reaction because pores of the metal-organic framework are used as active sites.
Methods for synthesizing core/shell structured metal-organic frame-works were proposed (Non-patent Document 3). The core and shell parts are made of the same metal and the organic ligands have the same size in order to maintain the lattices of metal-organic frameworks. Such synthetic methods are designed such that metal-organic frameworks can more efficiently store gases, such as hydrogen, carbon dioxide, and methane, further improvements are still needed.
Initial research on the synthesis of metal-organic frameworks has been directed to increasing the efficiency of metal-organic frameworks based on the use of new metals and organic ligands and further research has been conducted on the mechanism of gas storage for higher efficiency, leading to a better understanding of the mechanism of gas storage. Based on this research, the synthesis of structures in which micropores and mesopores coexist was reported (Non-patent Document 4). The coexistence of micropores and mesopores allows gas molecules to easily enter through the mesopore regions, enabling gas storage. However, the smaller micropores block the escape of the gas molecules in the larger mesopores (“self-sequestering”).
Metal-organic frameworks have very high capacities for gas storage. However, physisorption to pores having a large specific surface area limits practical applications of metal-organic frameworks to gas storages. To overcome this limitation, there is a need for a new synthetic method for metal-organic frameworks and a new gas storage mechanism.
The present inventors have found that when a host metal-organic framework and nano metal-organic frameworks including different metals and organic ligands are used, a metal-organic framework composite can be produced in which the nano metal-organic frameworks are embedded in the host metal-organic framework. The present inventors have also found that the metal-organic framework composite can be applied to gas storages with greatly improved gas storage efficiency. The present invention has been accomplished based on these findings.